Ozone levels over the Arctic have fallen dramatically this winter, say scientists.

An international group of researchers found cumulative ozone losses of more than 60% at around 18 kilometres (11 miles) above the polar region between January and March.

"These are among the largest chemical losses at this altitude observed during the last 10 years," said the European Commission, a main sponsor of the research, in a statement.

Ozone, a molecule in which three oxygen atoms are joined together, shuts out harmful ultraviolet radiation (less than 290 nanometres) coming from the Sun.

This radiation can damage DNA and lead to the formation of skin cancers.

'Not close to a hole'

EU spokeswoman Piia Huusela said the report did not point to a hole in the ozone layer such as the one that has opened over the Antarctic, but a weakening of ozone content in the stratosphere.

Ozone amounts over the Arctic today are now said to be 15% "below the pre-1976 average".

"This is not a hole in the ozone layer," said Piia Huusela. "We are not even close to a hole, but it is nevertheless alarming."

The results were obtained in the biggest study yet of ozone levels over the Arctic. The EU-sponsored Third European Stratospheric Experiment on Ozone (Theseo 2000) and Nasa's Sage III Ozone Loss and Validation Experiment (Solve) used a range of satellites, aircraft, balloons and ground-based instruments to collect data.

Much of the work was based near Kiruna, Sweden.

Recovery delayed

It has been known for some time that man-made chlorine and bromine compounds can upset the balance of chemistry at high altitudes leading to a thinning of the ozone layer.

Such compounds have been used as refrigerants, aerosol sprays, solvents and fire extinguishers, and world governments have now moved to restrict their use.

But this winter's ozone depletion reinforced concerns that recovery of the protective layer may be delayed, the scientists said.

The deterioration was partly due to colder temperatures in the stratosphere following a particularly severe winter. This may be caused by greenhouse gases, the by-products from burning fossil fuels which are also blamed for causing rising temperatures at the Earth's surface.

If this is the case, the scientists said, ozone levels would not recover immediately even when the more damaging chlorine and bromine compounds have disappeared from the atmosphere.